1. Field of the Invention
The present invention generally relates to a handover control method in a mobile communication system, more particularly to a handover control method and apparatus for varying a handover method according to an overlapping zone residence time of a handover-performing mobile station, and for varying a route of a handover path according to load on each node in a network.
2. Description of the Related Art
In a typical mobile communication system, an area within radio range of a radio base station is referred to as a cell, and there is a zone where the cells overlap each other, which is referred to as an overlapping zone hereinafter, as shown in FIG. 1. In FIG. 1, base station controllers are denoted by BSC, base transceiver stations are denoted by BTS, and a mobile station is denotes by MS. A handover process of the mobile station takes place when the mobile station comes into the overlapping zone.
Three methods for performing handover have mainly been proposed: (a) a gateway handover method as shown in FIG. 2, (b) a base station controller path extension handover method as shown in FIG. 3, and (c) a radio link path extension handover method as shown in FIG. 4.
In an existing mobile communication as typified by 3GPP (3rd Generation Partnership Project), it is common to fix a handover method for each network control method.
For example, in a circuit switching method in which a delay requirement is severe, (b) the base station controller path extension handover method may be employed, in which a path set up to the base station controller is extended, while in a packet exchange method in which a delay requirement is not so severe, (a) the gateway handover method may be employed, in which a path set up among relay nodes is reset up, in order to use network resources effectively.
Since the above-described conventional handover control method, however, fixes a handover method to each network control method, (1) the traffic is congested at a certain node when a certain network control method is intensively used in a certain area.
Particularly, in the conventional handover control method, since, upon a path being set up, paths via the same route are generally set up to the mobile stations using the same network control method in the same area, the above problem (1) is remarkable.
Also, since the above conventional handover control method determines the handover method to use according to the network control method, and does not take into account an overlapping zone residence time of the mobile station limiting a handover process time, (2) network resources are not used efficiently. This problem (2) is described in detail below.
The handover process must be completed while the mobile station stays in the overlapping zone. If the mobile station goes out from the overlapping zone without completing the handover process, packet losses or a line disconnection may occur because the mobile station has not established a connection with a handover-targeted base station.
As seen in FIG. 2, applying (a) the gateway handover method, both path routes before and after the handover process run through the same route to the gateway, and are adaptively routed in each optimal path route from the gateway to the mobile station, taking into account the load on each node. Also as seen in FIG. 3, applying (b) the base station controller path extension handover method, both paths before and after the handover process are routed in the same route to the base station controller, and are adaptively routed in each optimal path from the base station controller to the mobile station, taking into account the load on each node. Further as seen in FIG. 4, applying (c) the radio link path extension handover method, both paths before and after the handover process are routed in the same route to the previous base station before the handover process, and are adaptively routed in radio link paths via each optimal base station from the base station to the mobile station, taking into account the load on each base station.
Since the time required for the handover process is generally in proportion to the number of paths (i.e. hop count) to be reset up, comparing the handover process times among the above three methods, the handover process time in method (a) is longer than one in method (b), which is longer than one in method (c), in short, (a)>(b)>(c).
On the other hand, if a branch point where the path routes before and after the handover process go separate ways is located further downstream (closer to the mobile station) in the network, the paths fixed by the method increases and the flexibility of the path set-up decreases. In this case it becomes difficult to set up the optimal route for the network by taking into account the load on each node. Also, if the branch point is located further downstream in the network, a path route of a new connection may become devious, i.e. the path route may include redundant paths. In this case, because of such redundant paths, the efficiency of use of the network resources may decrease, and additionally the load on the network may increase. Therefore, as to the load on the network when the above three methods are employed, in general, the load with method (a) may be smaller than one with method (b), which may be smaller than one with method (c), in short, (a)<(b)<(c).
Thus, in selection of the handover method, the relationship between the handover process time and the efficiency of use of the network resources is a trade-off. Therefore, if it is intended to reduce the handover process time as a priority whenever the handover method is determined, it would not always be efficient from the point of view of use of the network resources.